I recommend the following book, which I have read and is still one of the best books on the subject of undergrad Ivey admissions:
A Is for Admission: The Insider's Guide to Getting into the Ivy League and Other Top Colleges
It is very eye opening. For example, it says the SAT Subject Tests (SAT II) can matter more than GPA. Another potential surprise is that the Ivies like to see work experience since it shows maturation. And, of course, being in a leadership position at high school club or other group is also helpful. Most of all, you need something that makes you stand out. This is referred to as the "
hook."